Paul Allen, who co-founded Microsoft with his childhood friend Bill Gates, has died aged 65.
Earlier this month, Mr Allen said the cancer he was treated for in 2009, non-Hodgkin’s lymphoma, had returned.
Mr Allen’s company Vulcan said in a statement that he died on Monday.
His sister, Jody Allen, said: “My brother was a remarkable individual on every level.
"Paul’s family and friends were blessed to experience his wit, warmth, his generosity and deep concern. For all the demands on his schedule, there was always time for family and friends."
He and Mr Gates founded Microsoft in 1975 and the company’s big break came in 1980, when IBM decided to move into personal computers.
IBM asked Microsoft to provide the operating system.
The decision thrust Microsoft on to the throne of technology and the two Seattle natives became billionaires.
Microsoft CEO Satya Nadella said: “Paul Allen’s contributions to our company, our industry and to our community are indispensable.
"As co-founder of Microsoft, in his own quiet and persistent way, he created magical products, experiences and institutions, and in doing so, he changed the world.
"I have learned so much from him – his inquisitiveness, curiosity and push for high standards are something that will continue to inspire me and all of us at Microsoft.
Both Mr Allen and Mr Gates later dedicated themselves to philanthropy.
Over the course of several decades, Mr Allen gave more than $2 billion to a wide range of interests, including ocean health, homelessness and advancing scientific research.
Mr Allen, who was an avid sports fan, owned the Portland Trail Blazers and the Seattle Seahawks.